29. All living organisms contain carbon. As organisms grow, they require
large quantities of carbon to carry out life processes and make new cells.
Where is the source of most of Earth's available carbon?

A. New carbon atoms are constantly being created by the process of
photosynthesis.
B. Carbon atoms are constantly transferred from the Sun to Earth along
with sunlight
C. Carbon is constantly transported from the mantle to the crust by volcanic
eruptions.
D. Carbon is constantly recycled from dead organisms back into the
environment.